Methods of anaerobic digestion of biomass to produce biogas

ABSTRACT

Improved methods for anaerobic digestion of organic matter to produce biogas. Among the improvements given are including ferric iron in a hydrolysis reactor to increase the rate and efficiency of anaerobic hydrolysis to provide substrates for methanogenesis. A solids separation step is added after hydrolysis and before methanogenesis to improve the efficiency of the methanogenesis step. Other improvements involve using separate tanks for the hydrolysis and methanogenesis stages and using two (or more) methanogenesis tanks in sequence, and switching the order of the two (or more) methanogenesis tanks periodically.

BACKGROUND

Anaerobic Digestion (AD) is a way of converting organic biomass tobiogas. Hazardous chemical waste can also often be digested bymicroorganisms and disposed of by fermentation.

AD produces biogas, a mixture of primarily methane and carbon dioxide,which can be used for energy. This is an advantage over landfillingwaste.

AD has three distinct phases: hydrolysis, acetogenesis andmethanogenesis. Hydrolysis and acetogenesis often occur in the same tankand are thus sometimes considered one phase.

In AD, anaerobic microorganisms ferment carbon substrates to products inthe absence of oxygen or oxygen surrogates. For instance, some organismstransform hexose sugars to ethanol and CO₂. Other common fermentationproducts include lactic acid, acetic acid, butyric acid, H₂, andmethane. Many of these compounds are themselves substrates for furtheranaerobic metabolism by other microorganisms. However, two fermentationproducts cannot be further fermented—methane and CO₂. Thus, allanaerobic decomposition can ultimately lead to methane and carbondioxide.

AD biologically is generally considered to occur in two phases: (1)Breakdown of sugars and carbohydrates to smaller molecules, particularlyorganic acids such as acetic acid and butyric acid. This is known ashydrolysis or sometimes as acid production. And (2) production ofmethane and CO₂ from the smaller organic molecules produced in phase(1), known as methanogenesis. Different organisms catalyze phases (1)and (2). Acid forming microorganisms and other microorganisms, thatinclude both facultative and obligate anaerobic microorganisms catalyzephase (1), the hydrolysis phase. Organisms that produce methane arecalled methanogens. Methanogens can produce methane from acetate by thereaction acetate+H₂O-->methane+HCO₃ ⁻. Methanogens can also producemethane from hydrogen and CO₂ by the reaction 4H₂+HCO₃ ⁻+H⁺-->CH₄+3 H₂O.The hydrogen for methanogenesis from carbon dioxide in nature comes fromfermentation of reduced carbon substrates.

Only methanogens—which are obligate anaerobes—produce methane, hydrogenand carbon dioxide through the cleaving of acetate and formate andremove protons from the cytoplasm.

Methanogenesis is the most essential part of AD as it is the only stagethat removes protons from the cytoplasm to allow the preceding steps toproceed. A well-functioning methanogenesis stage is key to maintaining afunctional and efficient AD systems. Without it AD systems become septicand fail.

Of all metabolic pathways, methanogenesis yields the least amount ofenergy as the end product—methane—has a high enthalpy. Consequentlythere is minimal energy yield for the organisms involved causing them togrow very slowly. System design to retain biomass is critical.Traditional AD systems accomplish stability through large tanks thatprovide adequate time for a stable methanogen population to bemaintained. This results in large tanks that are expensive to build andare not optimum.

Even in such tanks the process is always somewhat incomplete: someportion of the substrate is not digested all the way to methane and CO₂.And the speed of the process is a limiting factor that determines thesize of reactors needed. Thus, more efficient and faster methods ofanaerobic digestion are needed, and digester systems that facilitate orallow more efficient and faster anaerobic digestion are needed.

SUMMARY

The inventors have developed improved methods of fermentation thatferment organic substrates to biogas more quickly and more completelythan typical current methods.

One embodiment provides a method of producing biogas comprising:

(a)(i) grinding an organic substrate comprising solids to producesmaller solids particles (thus allowing for more intimate contact of thesubstrate with the microorganisms);

(a)(ii) transferring the substrate with smaller solids particles to ahydrolysis tank;

(a)(iii) hydrolyzing the substrate in an anaerobic condition in ahydrolysis tank comprising Fe³⁺ particles;

(a)(iv) transferring effluent from the hydrolysis tank to a solidsseparator

(a)(v) separating solids from the hydrolysis effluent to produce asolids fraction and a screened hydrolysis effluent;

(b)(i) transferring the screened hydrolysis effluent for a period oftime 1 to a methanogenesis tank 1 comprising fixed film methanogens orgranular methanogens and producing biogas and methanogenesis tank 1liquid effluent;

(b)(ii) transferring the methanogenesis tank 1 liquid effluent to amethanogenesis tank 2 comprising fixed film methanogens or granularmethanogens during period of time 1 and producing biogas andmethanogenesis tank 2 liquid effluent in methanogenesis tank 2;

(b)(iii) discharging methanogenesis tank 2 liquid effluent during periodof time 1; (c) and then switching the order of methanogenesis tanks 1and 2 and

(c)(i) transferring the screened hydrolysis effluent for a period oftime 2 to said methanogenesis tank 2 and producing biogas andmethanogenesis tank 2 liquid effluent;

(c)(ii) transferring the methanogenesis tank 2 liquid effluent to saidmethanogenesis tank 1 during period of time 2 and producing biogas andmethanogenesis tank 1 liquid effluent in methanogenesis tank 1;

(c)(iii) discharging methanogenesis tank 1 liquid effluent during periodof time 2.

In this embodiment, ferric iron is included in the hydrolysis tank.Typically it is fed daily to the hydrolysis tank. We have found that thepresence of Fe³⁺ particles increases the rate and efficiency of thehydrolysis stage, broadly defined as producing substrates formethanogenesis, which may include sugars, organic acids, H₂, and otherorganic molecules and inorganic molecules that are substrates formethanogenesis. The ferric iron functions as an electron sink orelectron acceptor. Other metallic electron acceptors may be used inplace of ferric iron.

The above embodiment also includes a solids separation step after thehydrolysis step to produce a screened hydrolysis effluent that we havefound is more suitable for the methanogenesis stage. We have found thatthis also improves the speed and efficiency of the methanogenesis step.The term “screened” hydrolysis effluent is used to mean that it isdepleted in solids and particles as compared to the hydrolysis effluentbefore the solids separator. The screened hydrolysis effluent may not betotally clear or totally free of solids and particles.

It further includes using separate tanks for the hydrolysis andmethanogenesis stages and using two (or more) methanogenesis tanks insequence, and switching the order of the two (or more) methanogenesistanks periodically. This is also found to improve the speed andefficiency of the process.

Another embodiment provides a method of producing biogas comprising:

(a)(i) transferring an organic substrate to a hydrolysis tank;

-   -   (a)(ii) hydrolyzing the substrate in anaerobic condition in a        hydrolysis tank to produce a hydrolysis liquid effluent;

(b)(i) transferring the hydrolysis effluent for a period of time 1 to amethanogenesis tank 1 comprising fixed film methanogens or granularmethanogens and producing biogas and methanogenesis tank 1 liquideffluent;

(b)(ii) transferring the methanogenesis tank 1 liquid effluent to amethanogenesis tank 2 comprising fixed film methanogens or granularmethanogens during period of time 1 and producing biogas andmethanogenesis tank 2 liquid effluent in methanogenesis tank 2; (b)(iii)discharging methanogenesis tank 2 liquid effluent during period of time1;

(c) and then switching the order of methanogenesis tanks 1 and 2 and

(c)(i) transferring the hydrolysis effluent for a period of time 2 tosaid methanogenesis tank 2 and producing biogas and methanogenesis tank2 liquid effluent;

(c)(ii) transferring the methanogenesis tank 2 liquid effluent to saidmethanogenesis tank 1 during period of time 2 and producing biogas andmethanogenesis tank 1 liquid effluent in methanogenesis tank 1;

(c)(iii) discharging methanogenesis tank 1 liquid effluent during periodof time 2;

(d) passing the biogas produced in methanogenesis tanks 1 and 2 througha foam trap to trap foam and separate it from the biogas; and

(e) collecting the biogas downstream of the foam trap.

The biogas is collected from both methanogenesis tanks 1 and 2. It isordinarily collected from the dome of the tanks, allowing the gas topass through a foam trap prior to the pressure equalizing tanks inpreparation for gas conditioning.

The process of anaerobic digestion often produces surfactants that causefoam to form. Foam in the biogas often causes many problems includingcorrosion and plugging of downstream gas units, especially when foam andvapor condenses at cooler temperatures. Including the foam trap in theprocess minimizes or eliminates these problems.

Another embodiment provides a method of producing biogas comprising:

(a)(i) transferring an organic substrate to a hydrolysis tank;

(a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysistank to produce a hydrolysis liquid effluent;

(b)(i) transferring the hydrolysis effluent for a period of time 1 to amethanogenesis tank 1 comprising fixed film methanogens or granularmethanogens and producing biogas and methanogenesis tank 1 liquideffluent;

(b)(ii) transferring biogas from the methanogenesis tank 1 through afoam trap to trap foam and separate it from the biogas; and

(c) collecting the biogas downstream of the foam trap.

Another embodiment provides a method of producing biogas comprising:

(a)(i) transferring an organic substrate to a hydrolysis tank;

(a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysistank to produce a hydrolysis liquid effluent;

(b)(i) transferring the hydrolysis effluent for a period of time 1 to amethanogenesis tank 1 comprising granulated and fixed film methanogensand producing biogas and methanogenesis tank 1 liquid effluent;

(b)(ii) recycling liquid in methanogenesis tank 1 to promote growth ofgranulated and attached biomass in the methanogenesis tank 1 reactors toimprove retention and reduce volume required for the methanogenesistank;

(b)(iii) transferring biogas from the methanogenesis tank 1 through afoam trap to trap residual foam and separate it from the biogas; and

(c) collecting the biogas downstream of the foam trap.

Another embodiment provides a method of producing biogas comprising:

(a)(i) transferring an organic substrate to a hydrolysis tank;

(a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysistank to produce a hydrolysis liquid effluent;

(b)(i) transferring the hydrolysis effluent for a period of time 1 to amethanogenesis tank 1 comprising fixed film methanogens or granularmethanogens and producing biogas and methanogenesis tank 1 liquideffluent;

wherein the hydrolysis tank and methanogenesis tank 1 each have arecirculation loop and one or more instrument detectors for monitoringone or more parameters of the liquid in the tank in the recirculationloop, and the method comprises recirculating liquid from each tank backinto the same tank through the recirculation loop for that tank, andmonitoring one or more parameters in each tank with the one or moreinstrument detectors in the recirculation loop of that tank, wherein theparameters are selected from the group consisting of pH, ORP, ionicstrength, chemical oxygen demand, and dissolved methane concentration;wherein each recirculation loop has at least one valve separating therecirculation loop from its tank, and the at least one valve can beclosed to allow the one or more instrument detectors to be removed forcleaning or service without allowing air to contact liquid in the tankconnected to the recirculation loop.

Another embodiment provides a method of producing biogas comprising:

(a)(i) transferring an organic substrate to a hydrolysis tank;

(a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysistank to produce a hydrolysis liquid effluent;

(b)(i) transferring the hydrolysis liquid effluent for a period of time1 to a methanogenesis tank 1 comprising fixed film methanogens orgranular methanogens and producing biogas and methanogenesis tank 1liquid effluent;

(b)(ii) transferring the methanogenesis tank 1 liquid effluent to amethanogenesis tank 2 comprising fixed film methanogens or granularmethanogens during period of time 1 and producing biogas andmethanogenesis tank 2 liquid effluent in methanogenesis tank 2;

(b)(iii) discharging methanogenesis tank 2 liquid effluent during periodof time 1;

(c) and then switching the order of methanogenesis tanks 1 and 2 and

(c)(i) transferring the liquid hydrolysis effluent for a period of time2 to said methanogenesis tank 2 and producing biogas and methanogenesistank 2 liquid effluent in methanogenesis tank 2;

(c)(ii) transferring the methanogenesis tank 2 liquid effluent to saidmethanogenesis tank 1 during period of time 2 and producing biogas andmethanogenesis tank 1 liquid effluent in methanogenesis tank 1;

(c)(iii) discharging methanogenesis tank 1 liquid effluent during periodof time 2.

In all these methods, the liquid in the hydrolysis tank is preferablyintermittently or continuously mixed. Mixing keeps any solids suspendedso they can be more efficiently digested and broken down. It is alsoimportant to periodically remove undigestible solids from the hydrolysistank. This can be accomplished by having the solids evenly suspendedwhen effluent is removed from the hydrolysis tank.

Another embodiment provides a mobile system for digesting organic matterand producing biogas, the system comprising: a shipping container ortrailer adapted for carriageon a truck or train; the shipping containeror trailer containing: (a) a digester comprising: (i) a pump for pumpinga liquid containing organic material into (ii) a hydrolysis tank; thehydrolysis tank hydraulically connected to (iii) a methanogenesis tankcomprising fixed film methanogens and an outlet for liquid effluent;(iv) a heater adapted for heating liquid in the container or liquid fedinto the hydrolysis tank or methanogenesis tank; (v) a plurality ofinstruments having detectors in contact with liquid or gas in thehydrolysis tank and methanogenesis tank; the instruments linked to a (b)computer for receiving data; and the computer linked to a (c) modem fortransmitting data from the shipping container or trailer to a remotecomputer.

Another embodiment provides a method of producing biogas comprising:heating clean water to steam or hot water; mixing the steam or hot waterwith an organic substrate; and fermenting the organic substrate tobiogas.

Heaters for anaerobic systems typically involve metal heating elementsin contact with the liquid containing substrate being digested. Theorganic matter in anaerobic digesters, including organic solids beingdigested, hydrolytic microorganisms, and methanogenic microorganisms,and other biomass, all can corrode heating elements and shorten thelives of heating elements. By heating clean water to produce hot wateror steam and mixing the hot water or steam with the an organic substrateto be digested, instead of heating the organic substrate directly, wehave been able to extend the life of the heating element.

Another embodiment provides a method of producing biogas comprising: (a)hydrolyzing an organic substrate in anaerobic condition in a hydrolysistank to produce a hydrolysis liquid effluent; (b) transferring thehydrolysis liquid effluent to a methanogenesis tank 1 comprising fixedfilm methanogens or granular methanogens and producing biogas andmethanogenesis tank 1 liquid effluent; wherein the methanogenesis tank 1has a liquid substrate and a recirculation loop for recirculating liquidsubstrate in the methanogenesis tank, the recirculation loop comprisinga heating element in contact with liquid substrate in the recirculationloop; (c) recirculating the liquid substrate through the recirculationloop and returning the liquid substrate to remainder of themethanogenesis tank; and (d) heating the liquid substrate in therecirculation loop with the heating element; wherein the liquidsubstrate in the recirculation loop is depleted in the fixed filmmethanogens or the granular methanogens compared to liquid substrate inthe remainder of the methanogenesis tank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the components of a digester system and a method ofproducing biogas.

FIG. 2 shows an anaerobic digestion tank with a recirculation loop and amonitoring instrument in the loop.

FIG. 3 shows methanogenesis tanks with a foam trap and components forcollecting biogas.

FIG. 4 shows a mobile system for digesting organic material andproducing biogas. The mobile system can be shipped to a distant site andused to test and demonstrate the methods of the invention on specificsubstrates at distant sites, while still being controlled and monitoredat a base location.

FIG. 5 shows use of a heater to heat feed water for mixing with the feedorganic substrate in the hydrolysis tank.

FIG. 6 shows use of a recycle flow and a heater element with amethanogenesis tank.

DETAILED DESCRIPTION

The inventors have developed new methods and systems for anaerobicdigestion of organic material to biogas.

One method and a system of the invention is shown in FIG. 1. It involvestransferring an organic substrate 1 to a hydrolysis tank 11. The organicsubstrate 1 can be any suitable organic substrate. In some cases it ischemical manufacturing waste. In other embodiments, it is agriculturalor food waste, for instance, potato skins, banana skins, food grease orfood oil, corn stover, etc. In other embodiments it is municipalgarbage, municipal waste, sewage, or livestock manure. In otherembodiments, it is slaughterhouse waste. It also may be, of course, acombination of wastes, which may include these wastes or others.

In FIG. 1, the organic substrate 1, where it includes solids, may beprocessed by grinding in grinder 2 to produce smaller solids to enhancehydrolysis. In a specific embodiment, the organic substrate 1 isprocessed by grinding to a particle size of 6 mm or less. In some cases,the organic substrate is or includes solids, in others it may be orinclude a liquid.

The organic substrate 1 is transferred to hydrolysis tank 11, where itis hydrolyzed in anaerobic conditions. The hydrolysis tank mayoptionally include ferric iron (Fe³⁺). The ferric iron can be in theform of magnetite or other forms of solid or dissolved ferric iron. Theferric iron is believed to improve the efficiency of anaerobichydrolysis because it functions as an electron sink. It is believed thatother metallic electron sinks (electron acceptors) could replace theferric iron. These would include, for instance, Mn, Co, Ni, Cu, and Zncations. In other embodiments, the hydrolysis tank does not includeferric iron or any other metallic cation electron sink.

In the hydrolysis tank the substrate is hydrolyzed and fermented tosmaller molecules that are substrates for methanogenesis. The content ofthe hydrolysis tank is preferably intermittently or continuously mixed.A hydrolysis effluent 12 is produced and is transferred to a firstmethanogenesis tank 21. The hydrolysis effluent 12 is preferablyprocessed by a solids separator 17 to separate out a solids fraction 14,which is typically removed from the system, and a screened hydrolysiseffluent 13. We have found that placement of a solid separator at thisstage improves the efficiency and speed of the methanogenesis stage.

The solids separator 17 in one embodiment is a screw type solidsseparator. Other types of solids separators may also be used.

In all the methods described herein, the liquid in the hydrolysis tankis preferably intermittently or continuously mixed. Mixing keeps anysolids suspended so they can be more efficiently digested and brokendown. It is also important to periodically remove undigestible solidsfrom the hydrolysis tank. This can be accomplished by having the solidsevenly suspended when effluent is removed from the hydrolysis tank.Selection of mixing intensity, frequency, and equipment used for mixingcan be optimized to effect conversion of the substrate or organic acidsand other soluble organic compounds.

The screened hydrolysis effluent is transferred to methanogenesis tank21. This is an anaerobic tank holding methanogens. Methanogens areobligate anaerobic microorganisms classified as archaea. Themethanogenesis tank preferably contains a substrate 41 on whichmethanogens can grow to form a fixed film 51. A good substrate is JAEGERSURFPAC. The methanogenesis tank 21 may contain granular bed methanogens52 in addition to fixed film methanogens 51 or instead of fixed filmmethanogens 51.

Methanogenesis tank 21 produces methanogenesis tank liquid effluent 21e.

The biogas from methanogenesis tank one (21) accumulates in a headspace25.

Methanogenesis tank one liquid effluent 21 e passes to methanogenesistank two (22). We have found that using two methanogenesis tanks insequence gives more complete digestion of the substrate and morecomplete conversion to biogas. Having a second methanogenesis tank alsohelps trap granules of methanogens to prevent loss of the methanogens.It helps trap the granules because the gas production is slower in thesecond methanogenesis tank since the substrate is depleted by the firstmethanogenesis tank. Less gas production means less bubbles trapped inthe granules and therefore the granules are less buoyant in the secondmethanogenesis tank and less likely to rise and be carried out in theliquid effluent 22 e.

Having recycle loops in each of the methanogenesis reactors (tanks) alsoserves to favor the growth of granulated and attached film biomass. Overtime, poorly settling fluffy biomass is removed while heavier granulesand film attached to the substrate for fixed film biomass is retained.

A liquid effluent 22 e is withdrawn from the second methanogenesis tank22 and biogas comprising CH₄ and CO₂ is also produced and collected.

The biogas in methanogenesis tank 22 also accumulates in a headspace 25before being collected or vented.

We have found that it is beneficial to switch the order ofmethanogenesis tank one (21) and methanogenesis tank two (22). Thisimproves the functioning of both tanks and improves the speed andefficiency of biogas production from the substrate. The second tank inorder receives less food (substrate for methanogens), so themicroorganisms would gradually die back if the tank remained in thesecond position permanently. By switching the order periodically, we canmaintain a high biomass density in the second tank that completes thedigestion of the feedstock to biogas with high yield. It also, asmentioned above helps to trap the granular bed methanogens in the secondmethanogenesis tank before they escape the system.

The methanogenesis tanks one and two (21 and 22) can be switched inorder at fixed periods of times, for instance every 24 hours or every 12hours. Alternatively, we have found good results by switching based onparameters that are easily measured using instruments. One suchmeasurement is the oxidation-reduction potential (ORP) of either or bothtanks. For example, the first methanogenesis tank in sequence (the leadmethanogenesis tank) should have an ORP of −300 to −550 mV. The ORP inthe lead methanogenesis tank rises over time, and it can be switchedwhen the ORP in it rises to a present value. For instance, the switchmay be made when the ORP in the lead methanogenesis tank rises to an ORPin the range of −300 to −400 mV. For instance, the preset value totrigger the switch may be a specific ORP between −300 and −400 mV, forinstance −350 mV. The second methanogenesis tank (the lag methanogenesistank) in sequence should have an ORP of about −300 to −550 mV. The ORPin the lag methanogenesis tank falls over time (becomes more negative).We have in some cases executed the switch when the ORP in the lagmethanogenesis tank falls to an ORP in the range of −450 mV to −550 mV.When the second reactor is moved to the first position, its ORP willrise because it will begin receiving hydrolysis tank effluent, which hasa higher ORP.

The specific ORP ranges listed in the foregoing description arepresented for illustration purposes and may vary from feedstock tofeedstock.

In a specific embodiment, the time period between switching the orderthe two methanogen tanks is between 6 and 24 hours inclusive. In anotherembodiment, it is between 6 and 48 hours, inclusive.

The hydrolysis tank 11, in addition to preferably containing Fe³⁺,preferably contains a microorganism that that reduces Fe³⁺ and producesat least one volatile organic acid from organic substrates. Thus, Fe³⁺is an electron acceptor for anaerobic respiration. This improves thespeed and efficiency of anaerobic digestion. The microorganism thatreduces Fe³⁺ and produces at least one volatile organic acid fromorganic substrates in one embodiment is or is derived from ATCC 55339.

Each of the tanks preferably has a recirculation loop, as shown in FIG.2 for hydrolysis tank 11. Recirculation loop 63 allows recirculation offluid in the tank. This is one way of maintaining an upward flow ofliquid in the main portion of the tank to stir the contents of the tankand maintain good mixing and less settling. This improves the efficiencyof the digestion. A recirculation pump 61 is coupled to recirculationloop 63 to pump liquid through the loop. In specific embodiments, therecirculation loop 63 includes one or more probes 62 to measure one ormore parameters of the liquid in the loop, such as pH, ORP, totalorganic carbon, and chemical oxygen demand. The liquid in therecirculation loop 63 of course is the same liquid in the tank 11, sothis allows one to measure these parameters in the tank. Therecirculation loop 63 can include valves 65 that can be shut off toseparate the recirculation loop. This allows us to remove the probe orprobes 62 for cleaning or calibration, without emptying the entirereactor and without exposing the contents of the tank to oxygen.

Another embodiment of the invention is the use of a pressure gauge nearthe bottom of the tank and a gas pressure gauge in the head space of thetank to measure liquid level in the tank. By the difference betweenpressure in the bottom of the tank and gas pressure above the liquidlevel, one can calculate the height of the liquid by use of a density of1.00 kg/L or whatever the density of the liquid in the tank actually is.Thus, another embodiment is a method of calculating height of liquid ina tank, wherein the tank comprises a liquid level and a gas headspaceabove the liquid level, the method comprising measuring liquid pressureat a known position A near the bottom of the tank and measuring gaspressure in a headspace above the liquid, and calculating the height ofthe liquid above the known position A by the difference in pressurebetween the gas pressure in the headspace and the liquid pressure atposition A.

In yet another embodiment, one or more hydrolysis tanks and/or one ormore methanogenesis tanks comprise a gas pressure probe in a headspaceof the tank and a liquid pressure probe near the bottom of the tank.

The hydrolysis tank 11 and first methanogenesis tank 21 and secondmethanogenesis tank 22 may each comprise a plurality of vessels,although they are each shown as a single vessel in FIG. 1.

Each of the reactors—the hydrolysis reactor, the methanogenesis reactor1 and methanogenesis reactor 2—has a hydraulic retention time, which isthe average time that the liquid is contained in the reactor. This isequal to the inflow flow rate (or outflow flow rate) divided by theliquid volume of the reactor. With the systems and methods describedherein, surprisingly small hydraulic retention times can be used whilestill achieving almost complete conversion of the substrate to biogas.In one typical embodiment, the hydrolysis tank has a hydraulic retentiontime of 2 days, and methanogenesis tanks 1 and 2 each have a hydraulicretention time of 1 day, and the entire system has a hydraulic retentiontime of 4 days.

In specific embodiments, the hydrolysis tank has a hydraulic retentiontime of 9 hours or less, 24-96 hours, 24-72 hours, 36-48 hours, or 72hours or less.

In specific embodiments, methanogenesis tank 1 and methanogenesis tank 2each have a hydraulic retention time of 48 hours or less, 12-48 hours,24-38 hours, 12-36 hours, 24-36 hours, or 12-24 hours.

The total hydraulic retention time is the hydraulic retention time forthe whole system, i.e., the sum of the hydraulic retention times of allhydrolysis tanks and methanogenesis tanks connected in series. Inspecific embodiments, the total hydraulic retention time is 8 days orless, 6 days or less, 4 days or less, about 4 days, 3-6 days, 4-8 days,3-8 days, or 2-8 days.

Referring to FIG. 3, the biogas accumulates in headspace 25 of the oneor more methanogenesis tanks 21 and 22. Where there are two or moremethanogenesis tanks, the headspace of the two or more tanks ispreferably connected, as shown in FIG. 3. There may be valves that canbe closed to separate the tanks.

The headspace biogas in one embodiment is collected through a foam trap62. The foam trap 62 has a large amount of surface area, on which foamand other liquids are trapped or condense.

The foam trap is advantageously used because foam can plug the gascollection system and can cause corrosion of system hardware.

Upstream from the foam trap, in some embodiments, may be a gas/solidsseparator 61. This also separates solids from the gas and separates asignificant amount of liquids and foam from the gas as well. Thegas/solids separator typically has an over-and-under flow pattern with awide section, then a narrow passageway, then another wide section.Solids and liquids tend to be trapped and fall back down in the narrowpassageway.

After the foam trap 62, the biogas is collected in biogas collector 63.This is preferably expandable to maintain the same gas pressure at alltimes.

In all the methods, systems, and devices described herein, themethanogens may be retained as fixed film methanogens or as granular bedmethanogens. In some embodiments, both fixed film and granular bedmethanogens are present.

FIG. 5 shows a system and method of the invention with hydrolysis tank11. Organic substrate 1 is shown being added to the hydrolysis tank 11for hydrolysis of the organic substrate 1. In FIG. 5, organic substrate1 also passes through a grinder 2 to grind solids to smaller solidsparticles for better hydrolysis and digestion of the solids. In FIG. 5 aheater 101 is shown. Water is heated by the heater 101 to produce hotwater or steam, and the hot water or steam is mixed with the organicsubstrate. The hot water or steam may be mixed with the organicsubstrate before it is added to the hydrolysis tank, as is shown in FIG.5, or can be mixed with the organic substrate in the hydrolysis tank.Either method accomplishes the goal of heating the organic substrateachieve an optimal temperature to promote faster and more efficienthydrolysis. With the method shown in FIG. 5, the heating element of theheater 101 only contacts clean water, not the organic substrate. Thisprolongs the life of the heater. Also, it may be advantageous to premixthe hot water or steam with the organic substrate before adding theorganic substrate to the hydrolysis tank 11. This prevents contactingthe microorganisms in the hydrolysis tank with extremely hot water orsteam, which would happen if the hot water or steam is mixed directlywith the contents of the hydrolysis tank, and which might kill some ofthe hydrolytic microorganisms. The term “clean water” in this context isintended to mean not that the water is necessarily absolutely pure, butthat it has less organic matter and other substances that can damage theheating element than the remainder of the “organic substrate.”

FIG. 6 shows the use of a heating element in a recirculation loopconnected with a methanogenesis tank. In FIG. 6, methanogenesis tank 22is shown with a recirculation loop 66. Liquid substrate 22 s of themethanogenesis tank is circulated through the recirculation loop 66, andit contacts a heating element 102 in the recirculation loop 66. Thisallows heating of the liquid substrate 22 s in the methanogenesis tankto an optimum temperature for methanogenesis. The recirculation loop 66is shown connected to the methanogenesis tank 22 near the upper level ofthe liquid substrate 22 s. Since the granules 52 and fixed filmmethanogens 51 are heavier than water, they will tend to fall in thetank and be depleted in the liquid substrate in the recirculation loop66 as compared to the liquid substrate in the remainder of themethanogenesis tank 22. This tends to save the heating element byminimizing its contact with solids and methanogenic granules andmicroorganisms.

The use of a recirculation flow as is shown in FIG. 6, with or without aheating element in the recirculation loop, is beneficial. By optimizingthe rate of recirculation, growth of granular methanogens is optimizedwhile preventing washout of the granular methanogens. Too rapid a flowcan lead to washout of the granular methanogens through liquid effluent22 e. But lower flow rates promote growth of granular methanogens.

FIG. 4 shows a mobile system of the invention for digesting organicmatter and producing biogas. The mobile system 70 comprises a shippingcontainer or trailer 71 adapted for carriage on a truck or train. Theshipping container or trailer contains: (a) a digester 72 comprising:(i) a pump 73 for pumping a liquid containing organic material into (ii)a hydrolysis tank 11; the hydrolysis tank 11 hydraulically connected to(iii) a methanogenesis tank 21 comprising fixed film methanogens. Themethanogenesis tank comprises (iv) an outlet 23 for liquid effluent. Thedigester further comprises (v) a heater 74 adapted for heating liquidcontained in or fed into the hydrolysis tank or methanogenesis tank; and(vi) a plurality of instruments 75 having detectors in contact withliquid or gas in the hydrolysis tank and methanogenesis tank. Theinstruments are linked to (b) a computer 76 for receiving data; and thecomputer linked to (c) a modem 77 for transmitting data 78 from theshipping container to a remote computer 81. The remote computer may beat a distance from the shipping container, that is, across the countryor the world.

The remote computer and the system may be configured to allow the remotecomputer to control and monitor the digester.

The mobile system may also include a solids separator operating betweenthe hydrolysis tank and the methanogenesis tank, as described above.

The mobile system allows the digester system to be transported by truckor rail to a distant site to test and demonstrate the performance of thesystem on a particular feedstock on location and under the conditionsfound at the location. The digester can be remotely monitored andcontrolled by persons at a distance so that those persons do not need totravel also to the site where the system is tested.

Example

The Novus Bio-Catalytic (NBC™) mobile pilot is a trailer-mounted,multi-cell, two stage anaerobic digestion system. The system currentlyconsists of 3 digestion cells, but has been designed to accommodate upto 7 cells. Each digestion cell is 4 feet in diameter (O.D.) and 9 feettall with a liquid capacity of 3,000 gallons (8′ liquid column) and agas headspace of 50.3 cubic ft. In the 3 cell configuration, liquidcapacity is 9,000 gallons. The pilot is housed in a semi-trailer with a50′×7′ box, ventilated and with electric baseboard heating. The pilot isdesigned to digest a variety of solid and liquid organic substrates. Inthe current 3 cell configuration, the digestion loading capacity of thesystem is:

1. Solids capacity: 1000 lbs (dry wt.) per day

2. Liquid Capacity: 500 gallons per day

The pilot system consists of the following:

1. Subsystem 1000: Feed Preparation and Pumping

2. Subsystem 2000: Hydrolysis

3. Subsystem 3000: Solids separation and recycle

4. Subsystem 4000: Methanogenesis and Polishing

5. Subsystem 5000: Gas Handling

The stages and their modes of operation are described in the followingsections.

Subsystem 1000: Feedstock Preparation and Mixing

The purpose of Subsystem 1000 is to convert solid and liquid substrateto a slurry suitable for hydrolytic digestion. The subsystem blendssolid and liquid substrates with magnetite into a slurry, reducing theparticulate size of the solid substrate so that it can pass through a ¼″opening and blend it with liquid waste and magnetite additive into aslurry. Stage Subsystem 1000 components include a feed pump (P1000)followed by a grinder with an open feed hopper for inlet.loop Subsystem1000 includes a influent pump to the grinder (40 gpm capacity), a feedwater heater (A.O. Smith model ATI 305201, 175,000 Btu/hour capacity),and a feed pump to the hydrolysis tank, and a feed make-up water tank,for water to add to the hydrolysis tank as needed.

Subsystem 2000: Hydrolysis

Subsystem 2000 solubilizes the solid particulate in the slurry to liquidorganic acids (hereinafter referred to as leachate) through thebiochemical and bacteriological catalysis of water and complex molecules(hydrolysis). The slurry is mixed and digested under anaerobicconditions at 35° C. to facilitate the converting the solids intovolatile acids. Feedstock slurry is pumped via P1000 into hydrolysistank TK2001, while leachate/slurry is pumped out of the tank via anotherpump. The hydrolysis tank (Ace Roto Mold VT1000-64, 1000 gallons). Thecontents in the hydrolysis tank are mixed by a ½ hp mixer with avariable motor. A discharge pump withdraws slurry from the top of thetank. Digestion parameters—[pH, ORP and Temperature]—are monitored byinstrument probes mounted in the recirculation pipe. Liquid level in thetank is monitored through a pressure element PE2001, set to maintain thepreset level. In addition, a high level alarm LAB 2001 shut down thepumps P1000. The pumps are then restarted only after intervention andreset by the operator.

Parameters monitored in Subsystem 2000 are:

Type of Parameter Control Input Output output Measured AIT 2001.1 PumpP2000 Digital pH On/Off signal AIT 2001.1 Pump P2000 Digital pH RPMPE2001 Cut off signal Digital Tank Level to P1000 LSH 2001/ High LevelDigital Tank Level PE2001 Alarm LAH PE2001 Cut off signal Digital TankLevel to P1000, Temperature Thermostat Digital Reactor TIT2001 ControlTemperature Signal AIT 2001.2 Record ORP Digital Reactor ORP

Subsystem 3000: Solids Separation and Recycle

Subsystem 3000 consists of the solids separation and leachate recyclesystems. The liquefied slurry from the hydrolysis tank is pumped to theSolids Separator (Vincent Corp. Model KP-10, 10 gpm capacity) using theCentrifugal Pump (American Machine Tool, AMPT-315-95A, 40 gpm capacity).Liquid leachate from the Separator is pumped to Methanogenesis Subsystem4000 on a continuous basis.

Subsystem 4000: Methanogenesis

Subsystem 4000 converts the leachate produced in Subsystem 2000(Hydrolysis) into biogas. The clarified leachate from the solidsseparator is collected by gravity into tank TK-4000 that serves as afeed pump station for this Subsystem. The leachate is pumped by pumpsP-4000.a or 4000.b The pump P-4000.a is oversized to keep the tankTK4000 mixed. TK4000 will have 2 chambers connected by an open loop atthe bottom. A portion of the flow will be directed into TK4001 or TK4002and P4000.b is a variable speed low flow pump. The leachate is passedthough loose fill media under strict anaerobic conditions at 35° C. tofacilitate the conversion of the acids into biogas. Leachate is pumpedinto the tank at the bottom and is removed at the top of the tank bygravity. TK4001 (methanogenesis tank 1) is mixed via recycle pump P4001,while TK4002 (methanogenesis tank 2) is mixed using pump P4002.

Both pumps withdraw leachate from the top and re-inject it at the bottomof their respective tanks. Digestion parameters—[pH, ORP andTemperature]—are monitored by instrument probes mounted on the dischargeloop of pumps P4001 and P4002.

Liquid level in the tanks is monitored through Pressure elements PE4001and P4002, set to maintain levels of 8′ in the tank, +/−3,″ by matchingthe rate of inflow and outflow: flow into the bottom of the tank viapump P4001, (and/or P4002) and 3 gph flow out at the top of TK4001(and/or TK4002) Outflow is determined by a gravity feed past a pre-setconstricted opening. As the liquid levels in TK4001 (and/or TK4002) riseto or drop below the preset levels as signaled by level sensor PE4001(or PE4002), the system starts or stops the pumps P4000_B and P5000 inorder to bring the liquid level back within range.

In addition, a high levels alarms LAH 4001 and LAH 4002 and a low levelalarm LAL 4000 in respective tanks TK4001 and TK4002 will shut downpumps P4000_B, and P5000 in the event the pre-set maximum or minimumlevels (in TK4000, TK4001 or in TK4002 is reached. The pumps will berestarted when the levels change.

Tanks TK4001 and TK4002 are designed to function in a seriesconfiguration wherein the lead and lag positions are switchedperiodically to achieve higher removal efficiencies and maintain morerobust bacteria colonies in both tanks. The switching is determined bythe ORP level in the lead tank.

The switching of the flow order of tanks TK4001 and TK4002 isaccomplished by controlling valves V4000.5, V4000.6, V4001.10 andV4001.26. The valve configurations for the two flow arrangements are asfollows:

Valve Valve Flow Regime 4000.5 Valve 4000.6 Valve 4001.26 4001.1026TK4001-TK4002 Open Open Closed Closed TK4002-TK4001 Closed Closed OpenOpen

Parameters monitored in Subsystem 4000 are:

Type of Control Input Output output AIT 4000.1 Pump P4000.1 DigitalStatus LSH 4000 High pump Digital level alarm Temperature ThermostatDigital TIT4001.1 Control Signal pH, AIT Low pH alarm Digital 4001.2 &Pump Speed Control Signal for pH below limits ORP, AIT High ORP Digital4001.3 signal to reverse flows PE4001 Start and stop Digital signal toP4000_A and P4000_B LSH 4001/ High Level Digital PE4001 Alarm LAH PE4002Cut off signal to Digital P4001 LSH 4002/ Low Level Digital PE4002 AlarmLAL PE4002 Cut off signal to Digital P4002 LSH 4002/ High Level DigitalPE4002 Alarm LAH PE4002 Cut off signal to Digital P4002, Close Valve ( )LSH 4002/ Low Level Digital PE4002 Alarm LAL pH, AIT Low pH alarmDigital 4002.2 & Pump Speed Control Signal for pH below limits ORP, AITHigh ORP Digital 4002.3 signal to reverse flows Temperature ThermostatDigital TIT4002.1 Control Signal pH, AIT Low pH alarm Digital 4002.2 &Pump Speed Control Signal for pH below limits ORP, AIT High ORP Digital4002.3 signal to reverse flows FE4000 Flow Signal Digital

Subsystem 5000: Gas Handling

The gas produced is measured and recorded on a continuous basis. Inaddition the system has an ambient gas level monitor mounted inside thetrailer outside the tanks for safety purposes. The trailer is equippedwith a large manometer consisting of 2 gas holding and 2 water balancingtanks to maintain positive pressure on the system as well as allow thetanks to be filled and emptied without introducing air. In essence themanometer functions like a floating cover.

What is claimed is: 1-17. (canceled)
 18. A mobile system for digestingorganic matter and producing biogas, the system comprising: a shippingcontainer or trailer adapted for carriage on a truck or train; theshipping container or trailer containing: a digester comprising: a pumpfor pumping a liquid containing organic material into a hydrolysis tank;the hydrolysis tank hydraulically connected to a methanogenesis tankcomprising fixed film methanogens and an outlet for liquid effluent; aheater adapted for heating liquid in the container or liquid fed intothe hydrolysis tank or methanogenesis tank; a plurality of instrumentshaving detectors in contact with liquid or gas in the hydrolysis tankand methanogenesis tank; the instruments linked to a computer forreceiving data; and the computer linked to a modem for transmitting datafrom the shipping container or trailer to a remote computer.
 19. Themobile system of claim 18 further comprising a remote computer at adifferent location outside of the shipping container or trailer; theremote computer adapted to control and monitor the digester.
 20. Themobile system of claim 18 wherein the methanogenesis tank comprisesfixed film methanogens and granular bed methanogens.
 21. The mobilesystem of claim 18 further comprising a solids separator to separateundigested solids, wherein the hydrolysis tank is hydraulicallyconnected to the solids separator, and the solids separator ishydraulically connected to the methanogenesis tank. 22-26. (canceled)